1. Field of the Invention
This invention relates to a multiple-layer microstrip or stripline assembly and, more specifically, to transitioning electrical signals between two or more separate layers. A microstrip is a type of waveguide comprising a conductor, typically in a flat, rectangular shape, separated from a single ground plane by a dielectric substrate material. A buried microstrip is a variation on the basic microstrip wherein the single conductor is embedded in a dielectric substrate material. A stripline is similar to a buried microstrip, except that a stripline has two ground planes--one located along each major surface of the dielectric substrate material.
2. Description of the Related Art
Many systems utilize multiple-layer microstrip assemblies in which various circuit elements are sandwiched between separate layers of dielectric material and laminated together to form a single composite structure. The types of various circuit elements to be used may include both passive and active components, as well as transmission lines (equivalently referred to as feedlines). One reason for using multiple-layer construction is to avoid electromagnetic interference between the signals present in the feedlines and circuit elements by isolating them from one another. In one frequently used configuration, the feedlines are arranged on the top surface of the bottom-most layer, separated by a dielectric substrate material from a ground plane located on the bottom surface of the bottom-most layer, while the various other circuit elements are distributed among the upper layers of the assembly.
When the feedlines and circuit elements which comprise a system are distributed over a plurality of separate layers, however, it becomes necessary to route signals back and forth between different layers to interconnect the various circuitry. For example, to implement a certain circuit function it may be necessary to connect a feedline on a first layer to a circuit element on a second layer. Additionally, i may be necessary to route several signals, originating on different layers, to appear on a single layer to facilitate connection to an external device.
The routing of signals between layers, however, presents problems. First, the process of constructing a microstrip assembly with inter-layer connections is time and labor intensive and burdensome due to the low tolerances for error. A known technique for constructing a multiple-layer microstrip assembly with inter-layer connections requires several steps as described below with reference to FIGS. 1A through 1E.
As illustrated in FIG. 1A, a ribbon 101 is connected by means of soldering to first top circuitry 103, for example, a circuit element or feedline, on a first layer 105.
Next, as shown in FIG. 1B, a second layer 107 must be brought into precise alignment with the first layer 105 such that the ribbon 101 may be passed through a hole 109 in the second layer 107. The task of aligning layers and feeding-through connecting ribbons requires a great deal of precision and is aggravated by the fact that, typically, several connections between the two layers, with each requiring a ribbon and alignment with a hole, must typically be made between the two layers.
As shown in FIG. 1C, the first layer 105 is then joined to the second layer 107 by means of a first laminate layer 111 therebetween. The first laminate layer 111 holds the ribbon 101 in a fixed position relative to the hole 109 in the second layer 107 and prevents disturbing the precise alignment achieved in the step performed as shown in FIG. 1B.
Next, as shown in FIG. 1D, the ribbon 101 passing through the hole 109 in the second layer 107 is connected by means of soldering to second top circuitry 113, for example, a circuit element or feedline, on the second layer 107. The ribbon 101 forms an ohmic electrical connection between the first top circuitry 103 on the first layer 105 and the second top circuitry 113 on the second layer 107 and completes the structure necessary to transition signals between analogous surfaces of two separate layers of a multiple-layer microstrip assembly.
Finally, as shown in FIG. 1E, an optional second laminate layer 115 may be disposed on top of the second layer 107 to cover the second top circuitry 113. The second laminate layer 115 insulates the second top circuitry 113 from unwanted ohmic short-circuits and holds the ribbon 101 firmly in position.
Although the above-described construction method provides a multiple-layer microstrip assembly capable of transitioning signals between separate layers, it has several drawbacks. Among the drawbacks, the prior method is time and labor intensive due to the need for a precise alignment step. Furthermore, the prior method presents a low tolerance for error due to the difficulty in aligning the small-sized hole and ribbon. Additionally, because the above-described structure requires a ribbon to be connected between two separate layers, the ribbon is subject to stress, possibly causing failure or an impedance mismatch, from any relative movement between the two layers during construction.
Another problem associated with routing signals between layers is that the interconnections may result in degradation of the signal due to reflections caused by impedance mismatches. It is for this reason that a prior multiple-layer microstrip assembly with inter-layer connections was likely to have diminished radio-frequency (RF) performance as compared to a single-layer microstrip assembly which did not require inter-layer connections.
Clearly, an apparatus and construction method for multi-layer microstrip assemblies that addresses these deficiencies is desirable.